BGA central ground matrix

All

It seems to me that when I learnt Kirchoffs there should have been a caveat that these laws are almost completely pointless (even at DC!)

Any decent pointers as to where to start reading would be appreciated.

Colin

Perhaps put some simulation results online, with an explanation of the input data. With simulation GIGO (garbage in, garbage out) easily comes into play.

I think the confusion is in the use of the word 'static'. A DC current produces a magnetic field (Ampere's law, and Maxwell's 4th eqn). Line up two conductors next to each other, run a current through them, and the two resultant magnetic fields will interact. An electron in bar A will move in the component of the magnetic field produced by bar B, and so will feel a force due to it. This is the proximity effect. Perhaps not all 'static', but all DC.

So, what's the answer? Either

a) The central balls "carry no current at all", are isolated from the die GND, and are just for thermal conduction, or

b) They are connected to the die GND, they do carry some return current, although less than the GND pins at the edge, and their decoupling is still important, but not very important?

I don't think he's talking about the magnetic field generating a DC current; but modifying the path of one that exists from other causes (the PSU).

Think about those moving electrons (beta particles) in a particle detector; a static magnetic field certainly modifies their path. (Thus you can determine their velocity from its radius)

Also think about the force on two busbars carrying a _DC_ current; on what does the force act? On the electrons, i.e. on the current, which (in modifying their path) apply force to the busbar.

The same will apply in a BGA package; it will not generate any current, but it may redistribute it between conductors.

- Brian

- Brian

I really regret I have to go back to this thread. I suggest everyone posting more on this nonsense makes sure to consult at least some high-school physics books first. The electrons (or beta particles, the origin does not matter) can be moved in vacuum or in a gas because they, when moving, produce a magnetic field, which interacts with the static magnetic field, exactly in the same way as two magnet pieces interact with each other (i.e. results in a force applied to the freely moving electron). When the electrons move inside a conductor (metal), this effect is seen as a mechanical force applied to the _conductor_. It takes electric rather than magnetic field to move electrons inside the conductor. This is how electric motors work. You _cannot_ affect the path of the electrons inside the conductor by a static magnetic field, just as you cannot force them to exit the conductor.

No. It cannot. I am tired of this thread, I would have hoped all electronics engineers would have at least some fundamental understanding of physics. Apparently there are some who don't. Anybody who has doubts please consider taking some basic course of physics.

Dimiter

------------------------------------------------------ Dimiter Popoff Transgalactic Instruments

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Hall effect.

-- Phil Hays

Hi Dimiter, Did you ever learn about the Hall Effect in your physics classes? Check out:-

Phil, you beat me by 3 minutes. Dammit! Cheers, Syms.

Hall effect has negligible values in conductors. On the other hand, it may take place somewhere on the die, that might well be. If the current does not flow out of the chip into the wires there is no need to redistribute it. I hope everyone is happy now.

Dimiter

Not always. Same as with skin effect, it isn't always negligible at power line frequencies. Best to do the calculations before, rather than after.

-- Caution: Contents may contain sarcasm.

It has nothing to do with frequencies, and we are talking DC. And yes, it is negligible inside the conductors in the context ot the thread, this is why I did not mention it at all. If you want to pursue this further, you are welcome to do the maths and prove me wrong, with all the sarcasm included. I shall not do it, I have more practical things to do.

Dimiter

Oh dear. Please excuse me.

I use several different signatures depending on the topic at hand. Please accept my statement that I didn't intend to use this signature for this discussion.

-- Phil Hays

Think about HOW that force is applied to the conductor.

Think about whether the presence of current (motion of electrons) is significant in this process, or whether, as your version suggests, the magnetic field and the conductor alone are sufficient.

That was never in debate. But once they are in motion, what happens?

- Brian

Uwe,

I am working on that.

Thanks.

Aust> aust>

Paul,

The latter (b).

They do carry current, but it is falling off as 1/r or 1/r^2 (I just can't remember which).

The BART rails had 2/3 nearest the power rail, and 1/3 in the rail furthest. Which makes me think it was 1/r, not 1/r^2.

Also, BART has shorting links every X meters that ties the two rails together (now) to lessen the return resistance (improve efficiency).

I think I was told that the inner 2X2 balls had 1/8 to 1/16 the current...but it may have been more (or less).

As I already said, I will post some results (when I find them).

Austin

Paul Johns> So, what's the answer? Either

If true, then why do superconductors have a critical magnetic field level. Moving electrons are influenced by magentic fields, the key question, is how much ?

-jg

I've given seminar talks for the last 20 years pressing designers to constantly reevalutate the underlying assumptions in a design, as they frequently change, and with small invalidations in the foundation, the whole design can, and does, fail. Actually to document them, and well.

As a consultant tackling failed projects, one reoccuring theme when I started probing the design/architecture was asking questions about the assumptions and getting the "everybody knows ..." answer. The "we have always done it that way, and it works ..." answer. Well, why is it now broke?

This is another case of "everybody knows", that will be fun to add to my on going talk, "It's not what you know that will hurt you, it's what you think you know" as a case study :)

Hmmm....

Hmmmm....

Please do, we can agree there is an effect, my antennae just question how much of an effect at DC ?.

You still have to satisfy ohms law, so any push effects that favour flow, have to model somehow as mV(uV) generators.... To skew Ball DC currents 7/8 or 15/16, frankly sounds implausible, and maybe the models there forgot to include resistance balancing effects ? [ ie do not believe everything you are 'told' ]

-jg